Fluoropolymers are a unique family of materials that are most often used where exceptionally high performance, maintenance-free, and long-lasting service-life is required. Among fluoropolymers, polyvinylidene fluoride (PVDF) has a great balance of properties and a long legacy as the only commercially accepted binder for LIB cathodes and as a separator in polymer battery. This usefulness comes from alternating CH2 and CF2 groups in PVDF backbone that produce a high dipole moment, resulting in good adhesion and compatibility with a vast array of materials.
U.S. Pat. No. 5,776,637 and U.S. Pat. No. 6,200,703 describe a PVDF binder solution in organic solvents, particularly in NMP, mixed with a powdery electrode material to form an electrode to be used in a non-aqueous-type battery. The role of the organic solvent is primarily to dissolve PVDF to provide good adhesion (non-reversible adhesion) and interconnectivity between the powdery electrode material particles upon evaporation of the organic solvent. The bonded powdery electrode materials together should be able to tolerate large volume expansion and contraction during charge and discharge cycles without losing interconnectivity within the electrodes. Interconnectivity of the active ingredients in an electrode is extremely important in battery performance, especially during charging and discharging cycles, as electrons must move across the electrode to current collector, and lithium ions must move within powdery particles of active materials as well as between anode and cathode. In order to achieve desired performance, PVDF binder is dissolved in a large volume of organic solvents, such as NMP with 20 to 1 ratio, and subsequently is admixed with powdery electrode forming material to produce slurry which upon casting and drying will form electrode.
An organic-solvent-based slurry presents safety, health and environmental dangers. Organic solvents are generally toxic and flammable, volatile in nature, and involve special manufacturing controls to mitigate and reduce environmental pollution and safety risks. Moreover, the large carbon footprint associated with use of organic solvents, is not environmentally desirable. Furthermore, extra manufacturing steps are associated with capturing and recycling large amount of NMP used in the preparation of slurry and fabrication of electrodes. A suitable waterborne fluoropolymer (particularly one that is PVDF-based), along with proper formulation could eliminate the need for large volume of organic solvents in the fabrication of electrodes for secondary Li-ion-batteries and overcome environmental hazards associated with use of such solvents.
There is an environmentally-driven, and safety-driven desire to be able to produce excellent, interconnected PVDF-based electrodes, without the massive use of organic solvents.
To effectively employ waterborne slurries in electrode-forming processes, it is important to develop binder systems that are compatible with current manufacturing practices and provide desired properties of the intermediate and final products. Some common criteria include: a) stability of the waterborne fluoropolymer dispersion, having sufficient shelf-life, b) stability of the slurry after admixing the powdery material, c) appropriate viscosity of the slurry to facilitate good aqueous casting, and d) sufficient interconnectivity within the electrode which is non-reversible after drying. Additionally, from a regulatory view, fluoropolymers made without fluorosurfactants are preferred.
U.S. Pat. No. 7,282,528 entitled “electrode additive” describes fluoropolymer dispersions for cathode electrodes, which are made by using per-fluorinated surfactants. The patent fails to teach or suggest the use of any fugitive adhesion promoters, and specifically the use of organic carbonates in the latex to provide and facilitate interconnectivity in the electrode that is non-reversible. Polytetrafluoroethylene (PTFE) binders, or blends of other fluoropolymers with 50% or more PTFE are preferred and exemplified. The negative electrode of the examples uses a conventional solvent-based PVDF solution.
U.S. Pat. No. 7,659,335 describes similar fluoropolymer dispersions used as electrode binders, having a specific class of non-ionic stabilizer used in post-polymerization. PTFE is either preferred since melt-processing or dissolution is substantially impossible. There is no mention of fugitive adhesion promoters or adding organic carbonates to the latex to provide interconnectivity within the electrode. There are large differences in the properties and processing of the final electrodes formed from PTFE and PVDF binders. PTFE polymers have very high melting points and exhibit very strong resistance to dissolution in common solvents. As a result, PTFE particles are not able to soften, flow, and adhere to powdery particles to provide interconnectivity within an electrode. Additionally, PTFE and its blends with other fluoropolymers do not meet some of the criteria needed as a viable binder, including the stability needed for waterborne fluoropolymer dispersion. Moreover, PTFE binders do not provide sufficient interconnectivity in electrodes which is non-reversible. Waterborne PVDF-based binders with organic carbonates of the present invention exhibit sufficient shelf stability, do not need concentrating steps, and provide interconnectivity when properly formulated.
A waterborne binder is described in US20100304270 for making an electrode using water as the media to prepare slurry instead of using conventional NMP solution. The disclosed slurry formulation requires anti-foaming agent, thickener, adhesion promoter, and a relatively high binder loading. In general, any additive used in the slurry formulation could have negative impact on the long-term performance of a lithium ion battery, because the additives could be oxidized in the cathode and generate off-gases. There is an interest in reducing the amount of non-active ingredients in a lithium ion battery in order to increase the energy density, so it is desirable to reduce the non-active materials in the slurry.
Surprisingly, it has been found that a mixture of a waterborne fluoropolymer with an organic carbonate as a fugitive adhesion promoter can provide an effective, economically friendly waterborne binder for a lithium ion battery. An especially useful organic carbonate is ethylene carbonate (EC) which is solid at room temperature, as the additive to binder media eliminates the need for organic solvent, wetting agent, or anti-foaming agent for slurry preparation. The waterborne fluoropolymer composition provides interconnectivity between active ingredients and sufficient adhesion to current collectors upon drying. Even though EC is solid at room temperature, it has been surprisingly found that the addition of EC to the waterborne fluoropolymer provides good interconnectivity and adhesion for powdery materials without a need for other additives.